Hydroxy functional polydiene polymers (polydiene diols) are well known. U.S. Pat. No. 5,393,843 discloses that formulations containing these polymers, a melamine resin, and an acid catalyst can be cured by baking under normal bake conditions. This same patent also discloses that these polymers can be mixed with isocyanates to yield compositions which cure at ambient temperature. It is known that, for instance, hydrogenated polybutadiene diols (EB diol) can be crosslinked by reaction with polyisocyanates at stoichiometry near 1/1 NCO/OH (NCO represents the isocyanate functionality which is active in the crosslinking reaction and OH represents the hydroxyl functionality). However, for economic reasons, it is only practical to make hydrogenated polydiene diols of relatively high hydroxyl equivalent weight (OH ew). However, these compositions based on hydroxy functional polydiene polymers cured with a crosslinking agent are usually fairly soft because the polymers are relatively high in hydroxyl equivalent weight, i.e., above about 750 ew OH (hydroxyl equivalent weight is the number average molecular weight divided by the number of functional groups per molecule) and thus are elastomeric and rubbery in nature and, although they can be very useful in some applications, they are too soft and too low in cohesive strength to be widely useful in applications such as hard coatings.
Attempts to increase hardness and adhesion by increasing crosslink density by simply mixing the polydiene diols and the polyisocyanate with a reinforcing agent, such as a low molecular weight diol or triol, were unsuccessful because the reinforcers are relatively polar and so are incompatible with the relatively nonpolar polydiene polymers. Incompatibility of the components leads to poor properties, such as gloss, in the cured composition or, even worse, the compositions may phase separate upon storage prior to cure.
I have discovered that this incompatibility problem can be overcome by synthesizing polyurethane resins based on a polydiene diol, a reinforcing diol or triol, and a polyisocyanate at appropriate NCO/OH ratios which are not close to 1:1. Essentially, this involves carrying out a limited reaction among the three components in order to render them compatible.